Subsurface drip disposal (“SDD”) systems are systems for disposing of wastewater such as septic tank effluent and the like. Subsurface drip disposal provides a shallow, slow rate, pressure-dosed system used for land application of pretreated wastewater. In general, SDD systems are characterized by: (1) uniform distribution of effluent, (2) dosing and resting cycles, and (3) very shallow placement of trenches. SDD systems typically use small diameter piping with subsurface drip emitters. The effluent must be adequately filtered before distribution through the underground emitter system, and filters and the piping network must be routinely flushed or otherwise cleared of trapped particles.
Well-designed SDD systems distribute effluent uniformly at a relatively low application rate over an absorption field, also called the drip field. Waste fluids are applied at a controlled rate in the plant root zone, which tends to minimize percolation of the effluent. Hydraulic loading rates may vary, for example between 0.1 and 1.6 gallons per day per square foot.
Conventional SDD systems include valving and control systems referred to as headworks that direct and control the flow of the effluent. In a conventional SDD system the headworks include two or three solenoid valves that control the timing and sequence of fluid flows. A conventional SDD system may include a tank wherein the effluent is accumulated for dispersal, a pump for removing effluent from the tank, a supply manifold and return manifold, and a number of emitter lines that extend between the supply and return manifolds and are disposed in the drip field. The emitter lines include a number of small emitters distributed along their length through which the effluent is dispersed in the drip field.
In a typical conventional SDD system the pump is periodically engaged, and a first solenoid valve is opened, to send flow to the drip field for dispersal through the drip emitters. A valve on the return manifold is typically closed such that the pumped fluid flows uniformly away from the tank, and is dispersed through the emitters. This is typically referred to as “dosing” cycle, and may occur, for example, twelve times a day, for periods of 5-10 minutes. It will be appreciated that this frequency and duration for the dosing cycle is by way of example, and the actual timing selected will depend on the particular application.
In order to avoid accumulation of matter in the emitter lines and manifolds, cooperatively referred to herein as the “field piping network”, a conventional SDD system will periodically engage a field piping network flushing cycle wherein relatively high-velocity effluent is pumped through the field piping network to clean out the pipes, with the valve for the return manifold open such that the effluent is partially returned to the tank. The minimum required fluid velocity for the flushing operation is often specified by local and/or state regulations. The field piping network flushing cycle may be engaged, for example, every 12-48 hours, and typically pressurizes the entire system such that effluent is also dispersed to the drip field, although the amount of such dosing is typically difficult to determine and/or unknown.
A conventional SDD system also includes a filter that prevents or reduces the amount of solid matter that is pumped from the SDD tank to the emitter lines, in order to prevent clogging of the emitters. Conventional SDD systems periodically engage a filter flush cycle wherein a third valve is opened to allow flow to go through a filter flushing port and return to the tank. In the filter flushing cycle fluids at a relatively high velocity are provided to remove matter from the filter. Typically the pump pressurizes the entire system, resulting in effluent also being dispersed through the emitters, although again the amount of fluid discharged may be difficult to determine or predict. In an exemplary septic tank application, a filter flush cycle may be engaged once for every 5-20 dose cycles.
SDD systems, particularly in cold weather climates, are designed and installed to drain back from the field piping (e.g., the emitters and plenums) into the dose tank after each dose, so that effluent does not freeze in the lines, potentially damaging the system. However, the headwords active valving systems used in most conventional SDD systems tend to interfere with proper drainage from the field piping, which can result in damage to the field piping network Moreover, the valving systems add significant costs and complexity to the drip disposal system. In addition, conventional SDD systems require three different pumping operations. There is one pressure and flow requirement for dosing the field, a different pressure and flow requirement for flushing the field piping network, and a third flow and pressure requirement for flushing the discharge filter. The differences in these three operating conditions make it difficult to select a suitable pump, and requires operation of the selected pump at non-optimal conditions at least some of the time.
To avoid the disadvantages associated with conventional SDD systems having headworks, a system without full headworks has been proposed in Design & Performance of Drip Dispersal Systems in Freezing Environments (published online at http://www.geoflow.com/research_w.html), by S. Wallace. However, the systems described therein include a solenoid valve for drain back of the effluent, and a throttle valve on the return head. Moreover, the disclosed system does not appear to include a filter, or means for flushing a filter.